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MMIO / memory-mapped I/O

Controlling hardware devices by reading and writing special memory addresses wired to the device instead of to RAM.

Rather than having separate instructions for talking to devices, the CPU maps a device's control registers into the ordinary address space, so writing to a particular address flips a hardware switch and reading it returns the device's status. This lets device drivers command hardware with the same load/store instructions used for memory. The catch is those addresses aren't real memory, so ordinary caching and reordering assumptions don't apply.